In multilayer coating systems a primer coating can be applied directly to a bare or pretreated metallic substrate to provide corrosion resistance. Chromate and other heavy metals have been used in primer coatings to improve the corrosion resistance and the adhesion of subsequently applied coatings. However, due to the hazards associated with chromate and other heavy metals, alternative non-chromate corrosion inhibitors have been developed.
For example, U.S. Pat. No. 8,231,970 describes the use of magnesium oxide such as magnesium oxide nanoparticles and other inorganic oxide nanoparticles as effective corrosion inhibitors for use in thermosetting polyamide/epoxy-functional polymer primer coatings. However, epoxy-based thermosetting primers tend to be unacceptably hard and brittle for certain applications such as aerospace coatings.
It is desirable to provide non-chromate primer coatings that exhibit the adhesion of thermosetting epoxy systems and the flexibility of polyurethane coatings.